I almost always claim more than I've purchased to be on the safe, what with me not being able to figure out an accurate exchange rate on the 2 or more countries(and currency's) I've been to on various trips. However on my last trip (MEX) claimed $0, brought nothing back, the only things I purchased where consumed in country (beer & way too much good food).
I didn't get a second look from those behind the desks and moved happily on my way.